Hang on thinking outside the box for a second how about adapting the
Barrett M97 Dragonfang, .50 Browning from the High Tech designers notes?
You'll need to add the stock back on, but you could do that by reversing the process in TS (and factor in the stat line is based on one handed firing)
That's a FA 12.7mm Acc 4*, assault carbine with a bulk of only -4*. It would be MinST14 (if I've factored two handed use and stock right).
It will still be heavy, but I'm not sure why your worrying too much about the weight at ST14+.
The range look's a touch high, but really it's not going to come up as you don't seem to be intending to fire this at long range anyway.
*when you put the stock on